It is well known that clean metal surfaces have a high surface energy which results in a strong gradient toward a lower energy state. Such surfaces easily adsorb gases such as oxygen which react chemically with strongly electropositive metal atoms to form a strongly bonded oxide layer on the surface (see Pask, Glass-Metal "Interfaces" and Bonding, U. of California, Lawrence Radiation Laboratory, Berkeley, Calif., Report UCRL 10611, 1963). By virtue of this mechanism, most metal surfaces which have been cleaned contain a layer of oxide. Such metal oxides have a considerably higher sintering temperature than the underlying metal. Thus, when mixtures of finely divided particles of metals are fired to effect alloying of the metals, it is found that the oxide layer interferes with the alloying function. This, in turn, results in significant quantities of unalloyed metal in the sintered body.
One field for the use of finely divided conductive metal particles in which the activity of the particles is of crucial importance is the fabrication of monolithic capacitors.
Monolithic capacitors comprise a plurality of dielectric layers, at least two of which bear metallizations (electrodes) in desired patterns. Such capacitors are made from a green (unfired) tape of particles of dielectric materials held together with an organic binder by cutting pieces of tape from a sheet of tape, metallizing some of the tape pieces, stacking and laminating the pieces of tape, cutting the laminates to form individual capacitors and firing the resultant capacitors to drive off any organic binders and vehicles and form a sintered (coherent) body.
Metallizations useful in producing electodes for capacitors normally comprise finely divided metal particles applied to dielectric green tapes in the form of a dispersion of such particles in an inert liquid organic medium or vehicle. Selection of the composition of the metal particles is usually based on a compromise of cost and performance. Performance usually requires the use of the noble metals because they are relatively inert during firing of the laminates to produce electrically continuous conductors. On the other hand, base metals often are oxidized in air at elevated temperatures and/or in many cases react with the dielectric material during firing.
As can be seen by reference to FIG. 1 of the drawing, the electrode layers of a typical multilayer capacitor unfortunately are not homogeneous and even, but are relatively porous. When all other parameters are the same, the capacitance of a particular multilayer capacitor is related to the area of the electrode layer. Therefore, to raise the capacitance for a given multilayer capacitor system, one must either increase the amount of the noble metal laydown to fill in the pore-like gaps in the electrode layer or improve the coverage of the electrode material over the dielectric material.